The present invention is directed to braking devices and, more particularly, to an apparatus for a disk brake.
Disk brake devices for braking vehicles, such as bicycles, operate by applying friction to a disk brake rotor that is non-rotatably mounted to a wheel. A well-known example of such a disk brake device is a piston retraction type brake device disclosed in Japanese Unexamined Patent Application 10-325432. In that device, a caliper has inner peripheral surfaces that form a pair of opposed cylindrical openings, a piston slidably mounted in each cylindrical opening, and an elastic piston ring seal that seals off the clearance space between the piston and the inner peripheral surface of the cylindrical opening. More specifically, each piston ring is mounted in an annular seal groove formed in the inner peripheral surface its associated cylinder. A movement restricting part is formed in the seal groove for restricting movement of an outer peripheral portion of the piston ring in the braking direction, and a movement permitting part is formed in the seal groove for permitting movement of an inner peripheral portion of the piston ring in the braking direction. The movement permitting part typically has the form of a surface that extends radially inwardly from the movement restricting part and inclines in the braking direction.
When the disk brake is activated, the piston moves in the braking direction to press a brake pad toward the disk brake rotor. The piston ring tends to move with the piston as a result of the frictional contact between the inner peripheral surface of the piston ring and the outer peripheral surface of the piston. However, such movement is generally blocked by the movement restricting part of the seal groove. On the other hand, an inner peripheral portion of the piston ring continues to move with the piston and bends toward the surface of the movement permitting part. The elastic resilience of the piston ring, and particularly the energy stored in the inner peripheral portion of the piston ring that bends toward the surface of the movement permitting part, is used to pull the piston away from the disk rotor when the brake is released.
Since the movement permitting part typically has the form of a surface that is inclined relative to the movement restricting part, the point of contact between the piston ring and the seal groove that functions as a fulcrum for the bending portion of the piston ring tends to move radially inwardly as the inner peripheral portion of the piston ring bends in the braking direction. This, in turn, causes the elastic modulus (spring constant) of the piston ring to increase, thus resulting in greater piston ring displacement relative to the applied load and a corresponding deterioration in brake feel due to variation in shoe clearance and brake release timing.